Zero recovery time pulse generator using polarity sensitive integrator driving schmitt trigger through cathode follower



3 o 6 09 u u n n M 0 NW E 0 2 9 INF 5T 83 3 o L L 1 m h m i E 55.358 EQEBS m ZERO RECOVERY TIME PULSE GENERATOR USING POLARITY SENSITIVE INTEGRATOR DRIVING SCHMITT TRIGGER THROUGH CATHODE FOLLOWER QMEMN Q E p M Q v L 58;; So 8 t N v. M65. 0 a h- M I 1 +m JOHN w. HAMMOND INVENTOR.

BY THOMAS J. HdLDE/V ATTORNEY 7., N 3034053 ZERO uncovnnv rnxrn iunsn GENERATOR US- ING PoLArurY snNsirrva INTEGRATOR DRIV This invention relates to video generators, and more particularly to video generators which will produce a video pulse of given width each time a trigger pulse is applied.

In simulating aircraft targets for a search radar device using a PPI type of presentation, angular gates and slant ranges of the simulated targets are supplied in the form of properly timed spike-like triggers to a video generator of the class described which, in response, supplies video pulses of given width that simulate target blips on the in dicator. When two targets have the same azimuth hearing, the video generator is supplied with triggers that are separated in time by an amount proportional to the distance between the targets. The ability of the indicator to resolve two targets at the same azimuth bearing thus depends upon the ability of the video generator to respond to trigger spikes which are almost coincident in time. Video generators of the class described usually employ blocking oscillators, monostable multivibrators, Schmitt circuits and the like, and include components which have a fixed recovery time after initial triggering and production of an initial video pulse and before a subsequent triggering is efiective to produce another video pulse. Consequently, a second trigger applied to the video generator during the time that its components are recovering from a first trigger cannot cause the video generator to produce a pulse representing a second target. The result of this situation can be illustrated by considering that two simultat'ed targets are approaching an apparent collision on a given azimuth bearing. On the PPI presentation, the two targets would approach, and at approximately the same slant range one target would disappear, namely the target more remote from the radar that would have been generated by the second trigger. In actuality, the targets might be suf ficiently far apart when they reach the approximate slant range that a collision would not occur, yet this cannot be simulated because of the finite recovery time of the components of the video generator. It has been suggested that a method of collision presentation or simulation can be achieved with conventional circuits by a trigger counting scheme in combination with several video generators and a video mixer.- The trigger countercircuit would select a video generator that is neither producing an output pulse nor recovering from such a pulse for application of a trigger. Where it is necessary to supply a series of successive triggers, the number of video gen- 'erators required increases as the time between successive triggers decreases. It is apparent then that a great deal of equipment is needed for collision simula ion, with the equipment being able to only approach zero time between successive triggers but never able to actually reach the goal.

Much effort has been expended by those skilled in the art to provide a video generator of the class described which may be retriggered at any time after the application of a trigger pulse, but so far as is known, the problems outlined above have not been solved. It is, therefore, an object of this invention to provide a video generator which has zero recovery time and which may be retriggered by successive pulses occurring at arbitrarily small time intervals, all without complicated circuits.

As a feature of this invention by which the objects thereof are achieved, a polarity sensitive pulse integrato is used to drive a regenerative pulse amplifier. The integrator has a capacitor which is instantaneously charged to a fixed value each time a trigger is applied so that the voltage is maintained above a given level for a definite time after receipt of a trigger, unless a subsequent trigger is applied, before the voltage drops below the given level. In the latter instance, the voltage is maintained above the given level for a definite time after receipt of the subsequent trigger. The amplifier includes a cathode-coupled binary circuit driven by a cathode follower, with the output pulses being derived from the plate of the normally conducting side of the binary circuit when such side is cut off. The cathode follower causes the binary circuit to switch, and the normally conducting side to remain cut oif as long as the grid of the cathode follower exceeds a given level. With this construction, the capacitor is free to be charged each time. a trigger is applied to the integrator regardless of the condition of the normally conducting side of the binary circuit. As a result, the time between output pulses can be made arbitrarily small, and an output pulse can be obtained even if a trigger is applied during the time that an output pulse is occurring.

The more important features of this invention have thus been outlined rather broadly. in order that the detailed description thereof that follows may be better understood,

and in order that the contribution to the art may be better appreciated. There are, of course, additional features of the invention that will be described hereinafter and which will also form the subject of the claims appended hereto. Those skilled in the the art will appreciate that the conception upon which this disclosure is based may readily be utilized as a basis for designing other structures for carrying out the several purposes of this invention. It is important, therefore, that the claims to be granted herein shall be of sufficient breadth to prevent the appropriation of this invention by those skilled in the art.

In the drawing:

FIGURE 1 is a schematic of a video generator made in accordance with this invention.

FIGURES 2' through 6 are voltage wave forms at various points in the generator. I v 7 Referring now to FIGURE 1, cathode-follower tube 10 has its grid 11 connected to receive a trigger through lead 12. Lead 12 is grounded through input resistor 1a which is shunted by Zener diode 14. Cathode 15 of tube 10 is connected to 'a source of negative voltage through cathode resistor 16, and plate 17 is connected to a source of B-| voltage. I

The output of tube 10 is derived from the cathode and is applied to polarity-sensitive integrator '20. The positive,

side of diode 2 1 of integrator 20 is connected at 22 to one side of variable resistor 23- and to the cathode of tube 10. The negative side of diode 21; is connected at 24 to the other side of variable resistor 23, and to grounded capacitor 25. Terminal Z4 is also connected to grid 31 of cathode follower tube 30. The cathode 32 of tube 39 is connected to a source of negative voltage through cathode resistor 33, while plate 34 is connected to B+'. The source of negative voltage on cathode resistor 16 insures that grid current never flows in tube 30 during any portion of the cycles described hereinafter inmore detail.

The output of tube '30 is derived from the cathode and is applied to a cathode-coupled binary 40 having tubes 50 and 6t Cathode 51 of tube 50 and cathode 61 of tube 69 are grounded through common cathode resistor 70. Grid 62 of tube to is connected to the output of cathode follower 313, and grid 52; of tube 50 is grounded through resistor 71. Plate 5 3- of tube 50 and plate 63 of tube 6% a 62 of tube 60.

.coupled binary 40 are suchthat tube 50 isnormally conpacitor 73. The pulse output is obtained at lead 74 from the plate of tube 50.. Those skilled in the art will recognize theoathode-coupled binary as a Schmitt circuit, the op,

eration of which is explained in pages 164m 173 of Pulse and Digital Circuits, Millman and Taub, McGraW-Hill Book Company, Inc, 1956;

The operation of the video generator shown in FIG- URE 1 will now be described. Consider that a positive going spike-like triggeris applied at input 12. Zener diode 14 serves to limit the amplitude of the spike-like trigger so that the input to grid 11 of tube is a trigger of fixed amplitude. The output of tube 10 is taken at the cathode so that the voltage at terminal 22 of polarity-sensitive time constant of integrator which is, of course, determined by the values of capacitor 25, variable resistor 23 integrator 20 follows the voltage at grid 11. The result is a trigger of fixed amplitude such as is illustrated at 100 in FIGURE 2.- Upon receipt of trigger 100, diode 21 conducts and instantaneously charges condenser to the peak amplitude of the output derived from cathode follower 10 as shown at 101 in FIGURE 3. At the termination of a trigger pulse, the decrease in voltage at terminal '22 causes diode 21 to cease'conduction and the voltage across condenser 25 discharges slowly through variable resistor 23 and cathode resistor 16 as shown at 102 in FIGURE 3. The term polarity-sensitive integrator when used herein means a circuit including an integrator and having a time constant that is extremely small when the input voltage increases (in either a positive or negative direction) in comparison to the time constant when, the input voltage decreases. Stated otherwise, the rise time of the output voltage in response to the leading edge of a very narrow trigger is so small in comparison to the decay time of the output voltage in response to the trailing edge of the trigger that," for practical purposes, the output voltage maybe considered as achieving the maximum amplitude of the input voltage substantially simultaneously therewith and then decaying exponentiallya An example of such circuit is shown as an R-C integrator network having the resistive element shunted by a diode whose forward resistance is very smallin comparison to the resistance of the element. I

The voltage at terminal 24 is coupled to grid 31 of cathode follower 30. By connecting the cathode of tube 10 to a source of negative potential, the maximum voltage attained by grid 31 is kept at such a value that grid current does not flow in tube during the time it is conducting. As a result, the wave form of the voltage at grid 31 is as shown in FIGURE 3 and is not distorted due to grid current flowing through resistors 16 and 23. The out-put of tube 30 follows the input andis coupled to grid The circuit constants of the cathodeducting and tube 60 is normally cut 011. In such condivoltage as shown at 103 in FIGURE 5, while the voltage at lead 74 due to conduction of tube 50 is lower than the B+ voltage and is shown at 104 in FlGURE 6. When grid 62 reaches cut-01f, tube 60 begins to conduct thereby lowering the voltage at grid 52, Due to a regenerative efiect arising from the coupling between tubes 50 and 60, conduction rapidly switches, and tube 50 is suddenlycut off as tube 60 begins to conduct heavily. Conduction of tube 60 continues as long as the voltage at grid 62 remains above a certain valueV, and when grid 62 drops below the vaIue'V, conduction suddenly switches back to tube 50. The source of negative voltage on cathodelresistor 33 insures'that tube60 of binary 40 is out 01f so that proper operation of the binary can be achieved.

and cathode resistor 16. Since the peak voltage at terminal 24 is determined by Zener diode 14 and is uniform from trigger to trigger, the voltage at terminal 24 will exceed V for a fixed period of time after application of a trigger. Consequently, the voltage at grid 62 will exceed V for the same period of time after application of a trigger. Such period of time can be varied by adjustment of variable resistor 23 to make the output pulse width of any desired value. When the voltage at terminal 24 drops below the value V and the voltage at grid 62 drops below the value V, binary 40 abruptly switches. The sudden conduction of tube 50 produces the trailing edge of pulse 105. The effect of a subsequent trigger 106 which follows initial trigger 100 by an amount'several times the output pulse width is shown to, produce output pulse 107, and

is the same as the effect of trigger 100.

When the time between two triggers is of the same order of magnitude asthe output pulse width, the effect of the second trigger can be understood by considering triggers 108 and 109. Upon the application of trigger 108, capaci-. tor 25 is charged to its peak value 101 whereupon the voltage at terminal. 25 decays exponentially below V' and towards its quiescent value at 110. Pulse 111 is formed by the conduction changes of tube 50 as previously described. However, before the voltage on capacitor reaches voltage at 110, trigger 109 is applied. Diode 21 again conducts so that the voltage on capacitor 25 is raised to its peak value 101, whereupon decay again occurs. Changes in conduction of tube 50 as a result of the voltage at terminal 24 produce output pulse 112 in the same manner that pulse 111 was formed.

Where a subsequent trigger follows an initial trigger by amount exactly equal'to the width of the output pulse of the generator, the eflfect is shown by considering triggers 113 and 114. In this case, trigger 114 causes the voltage at terminal 24 to rise to its peak value 101 as soon as the voltage has decayed to the value V'.' However, the voltage at grid 62 is still maintained above the value V and tube 60 remains conducting until the voltage at terminal 24 again reaches the value V. As a result, the two individual output pulses due to triggers 113 and 114 are combined to produce a single output pulse 115 whose duration is twice that of pulse 105. The effect of a subsequent trigger which occurs during the time that an output pulse is occurring can be seen by :referring to triggers 116and 117 and output pulse 118.

Those skilled in the art will now appreciate that this invention provides a video generator which may be retriggered immediately, since there are no components requiring recovery. During the output pulse, no current flows in grid 52 of the output tube of binary 50, hence commutation capacitor 73 is not charged in a manner requiring discharge before retriggering. Tube 30 isolates capacitor 25 from the effects of grid current flowing in tube 60 as the output pulse is occurring. Furthermore, capacitor 25 is free to accept additional charge at any time due to' diode 21, regardless of what is occurring in binary 40.. As a result of this construction, those skilled in the art will appreciate that none of the circuit components require recovery time after .an initial trigger is received and an initial output pulse is produced. Thus, the video generator has a zero recovery time and may be retriggered at any time after the application of an initial trigger.

What is claimed is:

1. A video generator for providing an output in response to positive triggers of uniform amplitude comprising a cathode follower driver stage for producing at its output positive pulses of uniform amplitude which follow the triggers applied to its input, a polarity-sensitive integrator responsive to the output of the cathode follower driver stage for producing an output which achieves the amplitude of the pulses substantially simultaneously with their application and then decays exponentially, a cathode coupled binary having an output terminal at a quiescent voltage, and means coupling the output of the integrator to said binary so that the voltage at the output terminal remains at a voltage other than the quiescentvalue as long as the output of the integrator exceeds a given value that is less than the amplitude of the pulses.

2. A video generator for producing output pulses in response to positive spikes of uniform amplitude comprising, a cathode follower driver stage for producing at its output positive pulses of uniform amplitude which follow the spikes applied to its input, a polarity-sensitive integrator responsive to the output of the cathode follower driver stage for producing an output which achieves the amplitude of the pulses substantially simultaneously with their application and then decays exponentially, the output of said integrator exceeding a given voltage for a fixed time, a cathode coupled binary having one circuit conducting when the other is cut oil, said binary being of the type in which said one circuit is rapidly cut off whenever said other circuit is caused to conduct and remains cut off as long as said other circuit is conducting, the output pulses being attained from said one circuit, conduction of said other circuit occurring when a voltage applied thereto exceeds said given value of voltage, and means connecting the output of the integrator with said other circuit so that said other circuit is caused to conduct as long as the applied voltage exceeds said given value. i

3. A video generator for producing output pulses in response to positive spikes of uniform amplitude comprising, a cathode follower driver stage for producing at its output positive pulses of uniform amplitude which follow the spikes applied to the input, a polarity-sensitive integrator responsive to the output of the cathode follower driver stage for producing an output which achieves the amplitude of the pulses substantially simultaneously with their application and then decays exponentially, the out-i put of said integrator exceeding a given voltage for a fixed time, a cathode follower isolation amplifier having an input circuit and an output circuit, the output of said integrator being applied to the input circuit of said amplifier, and a cathode coupled binary having a first circuit including a grid-controlled electron tube and having a second circuit, the grid of said tube being connected to the output of said cathode follower isolation amplifier, said grid causing said first circuit to be cut olf when the voltage on the grid is below a given value, said second circuit conducting when said first circuit is cut off, said grid causing said first circuit to conduct when the voltage on the grid is above said given value, grid current flowing through the cathode of the isolation amplifier when said first circuit conducts, said second circuit being cut off when said first circuit is conducting, output pulses being obtained from the second circuit when conduction switches between the circuits, said isolation amplifier maintaining the voltage on said grid below said given value at all times except during the time that the output of said integrator exceeds said given voltage, said voltage on said grid exceeding said given value whenever the output of the integrator exceeds said given value, said isolation amplifier serving to prevent the flow of grid current from affecting the output of the integrator.

4. In combination, a cathode coupled binary having an electron tube in each circuit, a control grid on one tube, said one tube conducting and the other tube being cut off only when the voltage at the grid of said one tube exceeds a given value, changes in voltage at the plate of said other tube arising from switches in conduction of said other tube producing output pulses whose width is determined by the time interval that the voltage at the grid of said one tube exceeds said given value, a cathode follower stage having a control grid, the grid of said one tube being connected to the cathode of said stage, the voltage at the cathode of said stage normally being less than said given value whereby said one tube is normally cut off and said other tube normally conducts, an integrator having its output connected to the grid of said stage, a cathode follower driver having its cathode connected to the input of the integrator, and a diode having its anode connected to the cathode of said driver and its cathode connected to the output of said integrator, the voltage at'the'cathode of said driver preventing said diode from conducting until a positive input spike is applied to the driver, an initial input spike causing said diode to conduct whereby the voltage at the grid of said stage achieves a peak amplitude substantially simultaneously with the spike and then decays exponentially, the peak amplitude of the voltage at the grid of said stage exceeding said given voltage, the voltage at the cathode of said stage following the voltage at the grid of said stage to a value no lower than said given value whereby conduction switches from said other tube tosaid one tube, conduction of said one tube being accompanied by the flow of grid current therein which limits the voltage at the cathode of said stage to a value no lower than said given value, flow of grid current continuing until the voltage at the grid of said stage decays to a value where the voltage at the cathode of said stage decreases below said given value whereby conduction switches from said one tube to said other tube, the voltage at the output of said integrator being independent of the voltage at the cathode of said stage whereby the application of a second spike to said driver will increase the voltage at the output of said integrator regardless of which of said tuba is conducting.

References Cited in the file of this patent UNITED STATES PATENTS 2,438,638 Lakatos Mar. 30, 1948 2,443,922 Moore June 22, 1948 2,511,093 Atwood June 13, 1950 2,653,237 Johnstone et a1. Sept. 22, 1953 2,741,742 Moore Apr. 10, 1956 2,802,101 West et al. Aug. 6, 1957 2,892,083 Norris June 23, 1959 

